This invention relates to an electrical machine employing a superconductive rotor, and more particularly to a method and apparatus for performing a vapor trap function within the pipe supplying liquid coolant to the rotor windings of superconductive machines.
It is a well known phenomenon that many metals, alloys and chemical compounds substantially lose all of their electrical resistance at temperatures near absolute zero. This phenomenon has highly advantageous applications when applied to electrical alternators or generators. To achieve the advantages resulting from superconductivity in the generator, and in particular, the electrical winding thereof, the winding must be operated below the critical temperature (above which the winding returns to its normal resistive conducting state). The critical temperature itself is a function of conductor current density and magnetic field strength. In general, the lower the temperature, the greater the current density and magnetic field may be.
In the past, it has been proposed to operate generators in their superconducting mode by submerging them in a liquid helium pool to keep the temperature of the winding below its critical temperature.
Generally speaking, the construction of a generator or alternator for superconductive operation entails the provision of a generally cylindrical, gas-tight outer shell that rotates with the shaft of the rotor. The electrical winding (hereinafter "winding") is disposed interiorly of and spaced a slight distance inwardly from the shell and rotates with the shaft. A quantity of helium is placed inside the shell. This quantity must be sufficient to fully submerge the winding in liquid helium when the generator rotates at its normal operating speed. During operation, the pool forms an inwardly facing liquid helium interface from which helium boils off into a gaseous center or core space of the rotor. Means must be provided to replenish the helium as it boils off and to keep the helium bath at sufficiently low temperatures so that the winding will remain below the critical temperature at all times.
Liquid helium is normally introduced into the shell interior via a liquid coolant supply pipe extending from the exterior of the shell to the core space. Typically the liquid coolant supply pipe is positioned interiorly within a central bore in one of the shafts. The supply pipe rotates with the shaft of the rotor. From the liquid coolant supply pipe the coolant flows into at least one coolant distribution conduit positioned interiorly of the shell and in fluid communication with the supply pipe.
Helium vapor is normally withdrawn through a similar bore at either end of the rotor for recirculation through a conventional refrigeration system.
Due to the importance of coolant flow in establishing and maintaining the generator in the superconducting state, flow regulation is instrumental in the continued development of this technology towards economic commercial application.
One group of regulators employs a coolant distribution conduit specifically designed to establish a hydrostatic bucking pressure against the inlet flow. This pressure regulates the flow of coolant into the rotor. One such regulator incorporates a "U" shaped tube which acts as a liquid trap. The bend in the tube is designed to maintain a quantity of the liquid coolant in the coolant distribution conduit at all times. This design thereby prevents coolant vapor present within the core from flowing into the liquid coolant supply pipe.
Examples of this type of flow regulation are found in the designs contained in the U.S. Pat. No. 4,048,529 issued on Sept. 13, 1977 to Pomeroy; and U.S. Pat. No. 4,091,298 issued on May 23, 1978 to Gamble.
Desirable is a compact design for a flow regulator which can accomplish the vapor trap function without suffering from resonant vibration or from material fatigue failure under transient, extreme conditions. Such a regulator should further be capable of delivering a controlled predetermined flow of liquid coolant over a wide range of operating conditions.